When installing various structures or constructions out at sea or in lakes, e.g. bridges, oil platforms or windmills etc, which are to be supported by an earth formation, or earth formations, at the bottom of a sea or lake, the process of sea piling may be used. Sea piling involves driving one or a plurality of piles (columns), e.g. made of steel, into an earth formation at the bottom of the sea by. The piles are often in the form of large hollow pipes, preferably with a circular cross-section, having an outer diameter of about 1-3 meters. In general, piles having a longitudinal extension which exceeds the depth of the sea are used during sea piling. The piles may be driven into the earth formation by means of an impact or percussion mechanism, which may be designed in various ways. The percussion mechanism may comprise an anvil, which is connected at the top end of the pile, and an impact or piling hammer arranged to strike the anvil and thus drive the bottom end of the pile into the earth formation at the bottom of the sea. The piling hammer may be hydraulically or pneumatically driven, e.g., or driven by other means.
A large piling hammer may have a weight of less than 200 tons. When a pile has been positioned substantially vertically in the water and the piling hammer strikes the top of the pile to drive the pile into the bottom of the sea, a pressure pulse is created which propagates through the pile and radiates sound or a water pressure pulse into the water. Generated water pressure pulses propagate through the water in a radial direction away from the pile. A large piling hammer striking a pile may in the water cause peak pressure levels in the range of 180-210 dB and Sound Exposure Levels, SEL, in the range of 150-180 dB at a radial distance of 750 meters from the pile. SEL is by definition the single pulse energy level integrated over one second. Over the last years, SEL measured at a radial distance of 750 meters has become the most common value to define the highest acceptable sound level in water. It is believed that high level sound or water pressure pulses, e.g. generated during sea piling, may have a negative effect on marine life/animals. Thus, there is an incentive to keep the water sound or noise levels at an acceptable level and authorities have begun to set water sound limits which are not to be exceeded during sea piling. 160 dB at a 750 meter radius from the sea piling event is an example of a requirement level which is not to be exceeded during sea piling.
Small piles having a relatively small outer diameter, i.e. less than 1 meter, can be piled with SEL below 160 dB at 750 meters radius without any additional or auxiliary sound mitigation measures. In general, larger piles, with an outer diameter greater than 1 meter, which is used for windmills and industrial sea platforms, require larger piling hammer input energy and cannot be piled with SEL below 160 dB at 750 meters radius without applying additional or auxiliary sound mitigation measures.
Presently, there is a plurality of known mitigation methods or techniques for attenuating water pressure pulses generated during sea piling, to reduce the SEL. In the low attenuation region, i.e. below 4 dB, structural pile damping and accurate piling management of piling hammer input energy versus soil resistance are sufficient in most cases.
Greater attenuation, i.e. 4-8 dB, is in general attained by a plurality of free air bubbles moving freely and applied as an air curtain around the pile.
For yet greater attenuation, i.e. 8-12 dB, a further developed technique using air bubbles has been suggested, which uses multiple air bubbles with specific diameters and enclosed in thin plastic shells mounted at specific positions in a large net. The net is suggested to be applied around the pile and enclose the pile.
Prior art mitigation methods or techniques have limitations which only allow a maximum attenuation of less than 12-15 dB, but that maximum attenuation for the prior art techniques requires an accurate or optimal piling process control and the usage of the most sophisticated techniques known. If greater attenuation is required, there is no safe and robust prior art technique.